Tables are for tabular data--designing with HTML/CSS isn't perfect, but using tables for layout is an even less-elegant solution. It leads to a markup soup of nested tags and tables and difficult-to-maintain codebase. Good design and good markup go hand-in-hand and often you need to strike a balance between flashy visual design and solid underlying structure. This is also why visual designers should understand how their designs get implemented in HTML and CSS--it will create a restraint and limit the amount of "hacks" you have to do to make a design work.

Also remember that not everyone views the web in a browser. Having well structured markup helps those that use screen readers or alternate methods of browsing the web.